Greetings,
Before the previous patch you could select an outer corner of a mine (see screenshot) to then reinforce that section.
Since the last patch, however, this isn't possible anymore. All I can do now is select the ground there and construct a supporting column. not a really big issue as that would do the trick as well, except that in some places in a mine those columns are just blocking passage when you go in with a cart or simply obstructing the placement of a cart.
For example, if you have mine shafts going in 4 directions.
If this is intentional: why?
If not: please fix this